dt-bindings: interrupt-controller: sifive,plic: Clarify the riscv,ndev meaning in PLIC

In PLIC, interrupt source 0 is reserved and should not be used.
Therefore, the valid interrupt sources are from 1 to riscv,ndev
inclusive.

Update the documentation to clarify this point.

[ tglx: Fixup subject prefix ]

Signed-off-by: Yangyu Chen <cyy@cyyself.name>
Signed-off-by: Thomas Gleixner <tglx@kernel.org>
Link: https://patch.msgid.link/tencent_720A4669773B1EE15EC720869C35C2F0490A@qq.com

Author: cyyself

Documentation/devicetree/bindings/interrupt-controller/sifive,plic-1.0.0.yaml

@@ -108,7 +108,9 @@ properties:
   riscv,ndev:
     $ref: /schemas/types.yaml#/definitions/uint32
     description:
-      Specifies how many external interrupts are supported by this controller.
+      Specifies how many external (device) interrupts are supported by this
+      controller.  Note that source 0 is reserved in PLIC, so the valid
+      interrupt sources are 1 to riscv,ndev inclusive.
 
   clocks: true